A member of the National Academy of Sciences, Meltzer researches the origins, antiquity, and adaptations of the first Americans who colonized the North American continent at the end of the Ice Age. He focuses on how these hunter-gatherers met the challenges of moving across and adapting to the vast, ecologically diverse landscape of Late Glacial North America during a time of significant climate change.

The Nature piece by Alexandra Witze focuses on Meltzer’s latest study to show that a comet, or any other kind of extraterrestrial impact, was not responsible for sudden climate change at the end of the Ice Age 12,800 years ago.

Proponents of the comet-impact theory have pointed to sedimentary deposits that they say prove that an object from outer space hit the Earth, killing the Clovis culture and causing the mass extinction of many animals.

By Alexandra Witze
Nature
One of the most controversial ideas about prehistoric North America — that an impact by an extraterrestrial object 12,800 years ago triggered a cold snap that killed off mammoths and decimated early human populations — is under fresh attack. Independent archaeologists have reanalysed the dates of geological material that reportedly represents the impact, and found that they do not match.

Supporters of the impact theory have put forth 29 sites, from North America to Europe and beyond, that contain a thin layer of sediments said to date to the start of the cosmic impact event. The latest study checked to see whether those sites were all really 12,800 years old.

Only 3 of the 29 are, the researchers report today in the Proceedings of the National Academy of Sciences1. The other sites either have not been dated using the usual radiometric methods, or are much older or younger than the reported impact. “The chronology doesn’t hold up,” says team leader David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas.

There is no doubt that something important happened in this region around 12,800 years ago. Temperatures in the Northern Hemisphere plummeted in a cold spell known as the Younger Dryas, and sophisticated hunters known as the Clovis people vanished from what is now the western United States. Many of North America’s famous large mammals, such as mammoths, went extinct

Carbon footprint
Impact proponents say that many lines of evidence point to a cosmic object crashing into Earth at the time2. These include reported tiny diamonds formed in the high pressure of an impact, and soot and charcoal from fires possibly triggered by the smash. Opponents counter that there are other explanations for these materials, and that a comet blast should have left a huge fingerprint in the geological record — but nothing of the sort has been found.

Meltzer’s team includes experts on North American Palaeoindians. “We know some of these sites, we’ve worked at some of these sites,” he says. “When we started to read the details [of the impact theory], it just didn’t add up.”

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